1. Field of the Invention
The present invention relates to a cation exchange membrane of a fluorinated polymer and electrolysis and a preparation thereof. More particularly, it relates to a cation exchange membrane of a fluorinated polymer having good electrochemical properties and mechanical properties and high chemical resistances such as chlorine resistance and alkaline resistance and thermal stability which is especially suitable as a cation exchange membrane for an electrolysis of an aqueous solution of an alkali metal chloride.
2. DESCRIPTION OF THE PRIOR ARTS
Recently, it has been proposed to use a cation exchange membrane for a production of an alkali metal hydroxide and chlorine in high purity instead of a conventional asbestos diaphragm for electrolysis.
It has been known that a membrane of a fluorinated polymer has desirable characteristics as the cation exchange membrane because of the requirements of high chemical resistances such as oxidation resistance, chlorine resistance and alkaline resistance and heat resistance.
The cation exchange membranes made of a copolymer of a fluorinated olefin and a fluorinated monomer having sulfonic acid group or a functional group which can be converted to sulfonic acid group such as the copolymer of CF.dbd.CF.sub.2 and CF.sub.2 .dbd.CFOCF.sub.2 CF(CF.sub.3)O(CF.sub.2).sub.2 SO.sub.2 F or a copolymer of a fluorinated olefin and a fluorinated monomer having carboxylic acid group or a functional group which can be converted to carboxylic acid group such as the copolymer of CF.sub.2 .dbd.CF.sub.2 and CF.sub.2 .dbd.CFO(CF.sub.2).sub.3 COOCH.sub.3 and the copolymer of CF.sub.2 .dbd.CF.sub.2 and CF.sub.2 .dbd.CFOCF.sub.2 CF(CF.sub.3)O(CF.sub.2).sub.3 COOCH.sub.3 have been known as the membranes having good electrolytic characteristics, and high chemical resistances such as chlorine resistance and alkaline resistance.
The former of the cation exchange membranes of the fluorinated polymer having sulfonic acid groups have been used as cation exchange membranes for electrolysis because the electrolysis can be performed in relatively high current efficiency under low electrolytic voltage by using the cation exchange membrane in an electrolysis for producing relatively low concentration such as 15 to 30% of alkali metal hydroxide.
However, when the cation exchange membrane of the fluorinated polymer having good electrochemical characteristics and high chemical resistances such as chlorine resistance and alkaline resistance, is used for the electrolysis, there is a disadvantage of unsatisfactory mechanical characteristics.
The cation exchange membrane contacts with an alkali metal hydroxide in relatively high concentration on one side and contacts with an alkali metal chloride in the other side in the electrolysis. Moreover, the different pressures of the different gases are respectively applied to the membrane, whereby the phenomena of partial swelling and contraction of the membrane are caused. Furthermore, the hardening of the membrane in ageing may be caused whereby the creases, crackings and laminal peeling of the membrane may be caused to form pin holes which cause the contamination of the raw material of an alkali metal chloride into the alkali metal hydroxide and the decrease of the current efficiency.
Heretofore, in order to improve the mechanical strength of the cation exchange membrane, it has been known to reinforce the membrane by disposing a reinforcing material such as fabric or unwoven fabric of polypropylene, polyvinyl chloride, polyvinyliden fluoride, polytetrafluoroethylene in the membrane.
In accordance with such manner, the dimensional stability of the membrane is improved however the mechanical strength against the severe condition caused in the electrolysis is not satisfactorily improved.
In said reinforcement, the electric characteristics such as the increase of electric resistance of the cation exchange membrane are deteriorated and the cost of the reinforcing material is expensive.